Ion implantation is a widely used technique for forming the many doped regions of a semiconductor device, e.g., source regions, drain regions, collector regions, emitter regions, etc. In general, ion implantation refers to a technique whereby ionized dopant atoms are accelerated into a surface of a semiconductor body at high energy, thereby driving the dopant atoms into the semiconductor body. The penetration depth of the dopant atoms depends upon a number of parameters including dopant type, semiconductor material structure and implantation energy. Once the dopant atoms are implanted into the semiconductor substrate, an activation step must be performed to electrically activate the implanted dopant atoms and to repair the crystalline damage in the implanted region. This is done by annealing the substrate (e.g., at temperatures of 700° C. or higher) to cause the implanted dopant atoms to move into substitutional lattice sites within the semiconductor body.
Laser thermal annealing (LTA) has developed as a promising technique for dopant activation. Generally speaking, laser thermal annealing refers to a technique whereby radiation from a beam of a laser source is directed into a selected portion of a semiconductor body. In contrast to a conventional furnace process, laser thermal annealing focuses the energy only on the selected portion. Thus, one particular portion can be annealed while the adjacent regions remain at or close to room temperature. One advantage of laser thermal annealing is a very high ratio of activation of the implanted dopant atoms. In fact, nearly perfect activation of the implanted dopant atoms (e.g., greater than 90%, 95% or even 99% activation) is possible using laser thermal annealing. Moreover, laser thermal annealing is very fast and requires a low thermal budget, as the laser quickly energizes precisely targeted regions while other regions of the semiconductor body are not energized.
One example of laser thermal annealing is described in U.S. Pat. No. 7,842,590 to Gutt et al., the content of which is incorporated by reference by its entirety. The Gutt document discloses a high-voltage diode with a structured p-doped region 5 disposed adjacent to an n-type field stop region 4. The structured p-doped region 5, among other things, improves the electrical performance of the device by providing compensating carriers during device commutation. Similar advantages are realized in an IGBT version of the device. The structured p-doped regions 5 are formed by implanting dopant atoms into the second surface 12 of the substrate. A laser thermal anneal is performed to activate the dopant atoms for the structured p-doped regions 5.
One drawback of the laser thermal annealing process described in Gutt is that perfect or near perfect activation of the implanted dopant atoms is only possible if the structured p-doped regions 5 are implanted relatively close to the second surface 12 of the substrate. That is, the laser thermal annealing process limits the implantation depth of the dopant atoms. For example, conventional laser thermal annealing techniques are only able to achieve perfect or near perfect activation of the implanted dopant atoms up to a depth of 400 nm (nanometers). In many cases, it is desirable to form doped regions beyond this depth of 400 nm. For example, in the case of the Gutt patent described above, it would be beneficial to form the at least a part of the structured p-doped regions 5 further than 400 nm second surface 12 so that a more robust emitter layer can be fabricated. Conventionally, this can only be done by some combination of processes that are costly and/or do not fully activate the implanted dopant atoms.